1. Field of Endeavor
The present disclosure relates to a motor driving apparatus, and more particularly to a driving apparatus for operating an interior permanent magnet synchronous motor at a speed higher than a rated speed.
2. Background
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.
A permanent magnet synchronous motor (PMSM) is a high-power and high-efficiency motor that is widely used as a traction motor in the fields of electric vehicles including hybrid vehicles, fuel cell vehicles, and the like, as well as other industries.
In particular, an interior permanent magnet synchronous motor (IPMSM) is a synchronous motor having a permanent magnet inserted into a rotor iron core. The IPMSM has excellent high-speed durability and drivability, and thus is suitable for use as an electric vehicle motor. In this application fields, the IPMSM is driven in a torque control mode, where vector control is performed for independently controlling a flux current and a torque current.
Furthermore, the IPMSM used for electric vehicles or hybrid vehicles is very wide in a driving speed range of a rotor to even include a flux weakening control region II in the driving regions. The flux weakening control region II defines a case where a center of a voltage limit ellipse of the IPMSM is positioned inside a current limit circle.
In the flux weakening control region II, only the voltage limit condition affects a driving limit condition of the IPMSM, and because size of DC-link voltage of an inverter is limited, maximum use of the voltage limit condition is advantageous in terms of output torque.
FIG. 1 is a block diagram illustrating a driving system of an interior permanent magnet synchronous motor according to prior art, where the system is driven by an inverter embodied by a vector control independently controlling a flux current and a torque current from an instruction torque.
The conventional driving system includes an inverter (101), an IPMSM (102) and a rotor position detector (103) attached to a rotor of the IPMSM.
The inverter (101) receives a command torque to output voltages (Vas, Vbs, Vcs) capable of being driven by the command torque, and the rotor position detector (103) calculates or measures a rotor position or a rotor speed.
The rotor position calculated or measured by the rotor position detector (103) is used for coordinate change by coordinate converters (106, 110), and the rotor speed is used by a current command generator (104).
The current command generator (104) outputs a current command on a synchronous reference frame in response to the command torque, the rotor speed, and the DC-link voltage of inverter. In case of IPMSM, the current command generator (104) generally uses two or more 2-D look-up tables, where the look-up table outputs d and q-axes current commands on synchronous reference frame relative to an entire driving region.
A current regulator (105) serves to control the current commands obtained from the current command generator (104) to output d and q-axes voltages on the synchronous reference frame.
The coordinate converter (106) uses the rotor position information obtained by the rotor position detector (103) to convert an output voltage of a current controller (105) to a voltage on a stationary reference frame.
A voltage limiter (107) uses an inscribed circle of a voltage limit hexagon to convert a voltage of the coordinate converter (106) to a voltage synthesizable by an inverter unit (108). The voltage limit condition of the voltage limiter (107) is determined by the DC-link voltage, and the voltage positioned at an outside of the inscribed circle of the voltage limit hexagon is prevented from being outputted and stays on the inscribed circle of the voltage limit hexagon.
The inverter unit (108) is a voltage type inverter including a power semiconductor such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), and supplies the voltage commands (Vas, Vbs, Vcs) for following a command torque to the IPMSM (102).
A current sensor (109) is interposed between the IPMSM (102) and the inverter (108) to measure a phase current applied to the IPMSM (102), and the current measured by the current sensor (109) is returned as a feedback to the current command generator (104) and the current controller (105) in response to the coordinate conversion of the coordinate converter (110).
FIG. 2 is an exemplary view illustrating a driving region of the IPMSM of FIG. 1, where A is a curve of a constant torque, and currents on d and q-axes on the synchronous reference frame relative to a constant command torque may have an infinite combination, B is a current limiting condition of inverter, and C, D and E are examples of voltage limit condition in response to rotor speed, where the voltage limit condition is changed by rotor speed, and an increased rotor speed reduces the size of a voltage limit ellipse to an F direction.
The sizes of d and q-axes currents on the synchronous reference frame controllable by the inverter (101) relative to the constant command torque are determined in a range satisfying both the current limit condition and the voltage limit condition. In a case a voltage margin is sufficient, the voltage limit condition is not affected by the limiting conditions, such that it would be advantageous to track a current command driving a MTPA (Maximum Torque Per Ampere) in terms of efficiency of IPMSM.
For example, in case a predetermined torque command of A is given, and a voltage limit condition is given as C, a current command to track a command torque is determined at G, where G is a driving point for satisfying the MTPA, and a region where only the current limit condition influences the driving point is defined as a constant torque region.
In a case a rotor speed increases to cause the voltage limit condition to move from C to D, a driving point moves from G to H along an arrow direction, because G is a current region uncontrollable by an inverter. Here, a region, where both the voltage limit condition and the current limit condition influence the driving point, as in the region where the driving point moves from G to H, is defined as a flux weakening control region I.
In a case a rotor speed further increases to cause the voltage limit condition to move from D to E, the current limit condition can no longer influence the driving region, and only the voltage limit condition can influence the driving point.
Here, a region where only the voltage limit condition influences the driving point is defined as a flux weakening control region II. The driving point at the flux weakening control region II moves from H to I along an arrow direction. Voltage Equations on the synchronous reference frame of IPMSM (102) are provided as below:
                              V          ds          r                =                                            R              s                        ⁢                          i              ds              r                                +                                    L              ds                        ⁢                                          ⅆ                                  i                  ds                  r                                                            ⅆ                t                                              -                                    ω              r                        ⁢                          λ              qs              r                                                          [                  Equation          ⁢                                          ⁢          1                ]                                          V          qs          r                =                                            R              s                        ⁢                          i              qs              r                                +                                    L              qs                        ⁢                                          ⅆ                                  i                  qs                  r                                                            ⅆ                t                                              -                                    ω              r                        ⁢                          λ              ds              r                                                          [                  Equation          ⁢                                          ⁢          2                ]            
where, a superscript ‘r’ is a synchronous reference frame, a subscript ‘s’ is a variable of stationary reference frame, ‘ωr’ is an angular velocity of rotor, ‘idsr’ and ‘irqsr’ are respectively stator d and q-axes currents on the synchronous reference frame, ‘Vdsr’ and ‘Vqsr’ are respectively stator d and q-axes voltages on the synchronous reference frame, ‘λdsr’ and ‘λrqsr’ are respectively stator d and q-axes rotor fluxes on the synchronous reference frame, Rs, Lds and Lqs are respectively stator resistance d and q-axes inductances.
A driving limit condition of IPMSM (102) is divided to a voltage limit condition and a current limit condition, and expressed as under:(Vdsr)2+(Vqsr)2≦(Vs,max)2  [Equation 3](Idsr)2+(Iqsr)2≦(Is,max)2  [Equation 4]where, Vs,max defines a size of maximum voltage synthesizable by the inverter (101), and Is,max defines a maximum or rated current of IPMSM (102). Vs,max is a maximum voltage synthesizable by the inverter (10) and influenced by the size of the DC-link voltage ‘Vdc’, and in a case the voltage limit condition is selected by the inscribed circle of voltage limit hexagon as in the voltage limiter (107) of FIG. 1, Vs,max may have the following value.
                              V                      a            ,            max                          =                              V                          d              ⁢                                                          ⁢              c                                            3                                              [                  Equation          ⁢                                          ⁢          5                ]            
As noted from the foregoing, the IPMSM (102) in the flux weakening control region II is driven at the MTPV (Maximum Torque Per Voltage) capable of outputting an available voltage at a maximum torque.
The moving process of current command is such that an inductance of the IPMSM (102) is saturated by the size of the current to have a non-linear relationship. Thus, the driving of the IPMSM is such that characteristic of IPMSM is measured in advance (off-line) to prepare at least two or more 2-D look-up tables, whereby the current command generator (104) of FIG. 1 generates a current command on the synchronous reference frame responsive to the constant torque, the driving speed and the DC-link voltage.
The 2-D look-up table uses the torque command and the flux information as input to generate the d and q-axes current command on the synchronous reference frame. At this time, the flux information is obtained by dividing the DC-link voltage by rotor speed.
FIG. 3 is a schematic view illustrating a 2-D look-up table according to prior art. Referring to FIG. 3, the 2-D look-up tables (301, 302) receive the command torque and an input from a flux calculation unit (303) to output the d and q-axes current commands on the synchronous reference frame.
A feedback current of the current command generator (104) of FIG. 1 and the coordinate converter (110) is inputted to the current limiter (105). The current limiter (105) is a proportional and integral controller to synthesize an output voltage as per the following Equations.
                              V          ds                      r            *                          =                                            (                                                K                  pd                                +                                                      K                    id                                    s                                            )                        ⁢                          (                                                i                                      ds                    -                    ref                                    r                                -                                  i                  ds                  r                                            )                                -                                    ω              r                        ⁢                          λ              qs              r                                                          [                  Equation          ⁢                                          ⁢          6                ]                                          V          qs                      r            *                          =                                            (                                                K                  pd                                +                                                      K                    iq                                    s                                            )                        ⁢                          (                                                i                                      qs                    -                    ref                                    r                                -                                  i                  qs                  r                                            )                                -                                    ω              r                        ⁢                          λ              ds              r                                                          [                  Equation          ⁢                                          ⁢          7                ]            
The coordinate converter (106) converts an output voltage of the current limiter (105) on the synchronous reference frame to a voltage on the stationary reference frame using the following Equations.Vdss*=Vdsr*cos θ−Vqsr*sin θ  [Equation 8]Vqss*=Vdsr*cos θ+Vqsr*sin θ  [Equation 9]The voltage limiter (107) limits a voltage of the coordinate converter (106) and outputs the voltage, so that a voltage command can exist within the inscribed circle of the voltage limit condition expressed by a hexagon on the stationary reference frame, and the inverter unit (108) synthesize a voltage of the following Equations from the voltage limiter (107) and supplies the voltage to the IPMSM (102).Vas=Vdss  [Equation 10]
                              V          bs                =                                            -                              1                2                                      ⁢                          V              ds              s                                +                                                    3                            2                        ⁢                          V              qs              s                                                          [                  Equation          ⁢                                          ⁢          11                ]                                          V          cs                =                                            -                              1                2                                      ⁢                          V              ds              s                                -                                                    3                            2                        ⁢                          V              qs              s                                                          [                  Equation          ⁢                                          ⁢          12                ]            
Current sensors (109a-109c) measure a phase current between the inverter unit (108) and the IPMSM (102). The coordinate converter (110) converts the phase current to a current on the synchronous reference frame using the following
Equations and provides the current to the current limiter (105) as a feedback.
                              I          ds          s                =                                            2              ⁢                              i                as                                      -                          i              bs                        -                          i              cs                                3                                    [                  Equation          ⁢                                          ⁢          13                ]                                          I          qs          s                =                                            i              bs                        -                          i              cs                                2                                    [                  Equation          ⁢                                          ⁢          14                ]            idsr=idss cos θ+iqss sin θ  [Equation 15]iqsr=−idss sin θ+iqss cos θ  [Equation 16]
However, there is a problem in that performance of the IPMSM driving system of FIG. 1 deteriorates, because the current command generator (104) uses a pre-measured look-up table to cause subject parameters of the IPMSM to change.
Furthermore, there is another problem in that, even if the subject parameters of the IPMSM are not changed, the driving performance of motor is determined by performance of the look-up table, because the look-up table determines the performance of an entire driving region.
There is still another problem in that a voltage utilization rate of the inverter decreases to thereby decrease the output torque, because amount of voltage synthesized by the inverter is limited by the inscribed circle of the voltage limit hexagon.
It is, therefore, desirable to overcome the above problems and others by providing an improved apparatus for operating the interior permanent magnet synchronous motor.